Globular Clusters

Globular clusters (GCs) are among the oldest stellar components in galaxies. Their stellar populations have been widely recognized as holding important clues about the formation of both GCs and their host galaxy. It is believed that GCs formed in episodes of intense star formation, i.e. the major star-forming episodes that shaped galaxies (see Brodie & Strader 2006, and references therein). Therefore, the properties of both systems, especially their metallicities, are correlated (e.g. Brodie & Strader 2006).

Our present understanding of massive galaxy formation suggests a two-step scenario: the central part is the relic of the early stages of formation, and their extended halo results from minor merging (e.g. Trujillo, Ferreras & de La Rosa 2011). In this sense, GC systems may provide constraints for this scenario. For example, the presence of GC formation may help to distinguish between two types of minor mergers in a galaxy: dry (little or no gas) or wet (with gas).

We conducted a comprehensive multiwavelength photometric analysis of the innermost (∼3 × 3 kpc2) 110 globular clusters (GCs) of M87. Their spectral energy distributions (SEDs) were built taking advantage of new ground-based high-resolution near-infrared imaging aided by adaptive optics at the Very Large Telescope combined with Hubble Space Telescope ultraviolet–optical archival data. These GC SEDs are among the best photometrically sampled extragalactic GC SEDs. To compare with our SEDs we constructed equally sampled SEDs of Milky Way GCs. Using both these Milky Way cluster templates and different stellar population models, ages of >10 Gyr and metallicities of [Fe/H] ∼−0.6 dex are consistently inferred for the inner GCs of M87. In addition, the metallicity of these GCs is low (Δ[Fe/H] ∼0.8 dex) compared to that of their host galaxy. These results agree with the idea that the GC formation in M87 ceased earlier than that of the bulk of the stars of the central part of the galaxy. The ages of the inner GCs of M87 support the idea that these central parts of the galaxy formed first. Our data do not support evidence of recent wet merging.

SEDs of a subsample of four M87 GCs

Comparison of the average SED for M87 GCs with some Milky Way clusters. The metallicities ([Fe/H]) for the Milky Way GCs are −2.14 for NGC 2419 (purple), −1.80 for NGC 4147 (blue), −0.70 for 47 Tuc (green), −0.10 for NGC 6528 (red) and +0.45 for NGC 6791 (yellow). The solid orange polygon corresponds to the average template SED for M87 GCs. Bottom panel: residuals of the difference between the average of M87 GCs and the Milky Way clusters. The mean metallicity for M87 GCs is Z ∼ 0.004 ([Fe/H] ∼ −0.7).

Distribution of ages and metallicities for the 13 GCs with NIR photometry. In each panel, the red star corresponds to the mean age and metallicity, respectively.